The present invention relates to an apparatus for detecting whether a weft yarn is inserted in a warp shed in a jet loom such as a water jet loom or an air jet loom, and more particularly to electric adjusting means in such a jet loom for automatically increasing the gain of an amplifier connected to a weft feeler head as the sensitivity of the latter is reduced.
Air jet looms incorporate a photoelectric feeler head for detecting whether a weft yarn is properly inserted in a warp shed. The photoelectric feeler head comprises a light-emitting diode disposed at an end of the warp shed and a phototransistor positioned in confronting relation to the light-emitting diode. Any change in the amount of light from the light-emitting diode to the phototransistor due to an inserted weft yarn is sensed by the photoelectric feeler head to determine whether the weft yarn insertion is proper or not. If a mass of fly waste is accidentally attached to the lens in a light transmission window of the light-emitting diode or the phototransistor, then the detecting sensitivity of the feeler head is lowered dependent on the amount of waste meaterial attached to the lens. One solution has been to increase the feeler head sensitivity in advance to compensate for a sensitivity reduction at a later time. However, signals from the feeler head become saturated during a period in which the feeler head lenses suffer from a relatively small amount of fly waste. A malfunction may also be caused by a mass of fly waste which has just passed through the feeler head. For the reasons described above, it has been difficult for the conventional photoelectric feeler heads to keep a desired degree of weft sensitivity.
The applicant has proposed a weft detection process in which an optimum weft sensitivity is established initially for a feeler head, and a reduction in the level of feeler signals at a later time is detected to thereby give an alarm. This arrangement has allowed the feeler head to operate with fewer malfunctions, but has failed to increase the period of time in which the feeler head remains capable of operating properly.
Water jet looms have an electrode feeler head for detecting whether a weft yarn is inserted properly in a warp shed. The electrode feeler head comprises a pair of electrodes connected to a DC power supply for generating an electric signal when an inserted weft yarn is brought into contact with the electrodes. A problem with the electrode feeler head is that the insulation between the electrodes becomes deteriorated during use and weft yarns cannot be detected with sufficient sensitivity due to a leakage current.
Since both the photoelectric and electrode feeler heads are therefore subjected to a reduction in the weft sensitivity with time, with the result that they fail to detect weft yarns under stable conditions for an extended period of time.